1. Field of the Invention
This invention relates generally to bandgap reference circuits and more particularly to CMOS bandgap reference circuits.
2. Description of the Prior Art
Typically, the best reference for a good reproducible, stable voltage below three volts has been the bandgap reference circuit. As discussed in Analysis and Design of Analog Integrated Circuits by Paul R. Gray and Robert G. Meyer (John Wiley and Sons, 1977, pages 239-261), the base to emitter voltage V.sub.be, of a bipolar transistor exhibits a negative temperature coefficient with respect to temperature. On the other hand, R. J. Widlar has shown that the difference of base to emitter voltages .DELTA.V.sub.be of two bipolar transistors exhibits a positive temperature coefficient with respect to temperature. Thus, the sum of the base to emitter voltage, V.sub.be, of a bipolar transistor and a differential voltage .DELTA.V.sub.be will be relatively independent of temperature when the sum voltage equals the energy gap of silicon. Such temperature stable references have been created by generating a V.sub.be and summing a .DELTA.V.sub.be of such value that the sum substantially equals the bandgap voltage of 1.205 volts.
A standard CMOS process can be used to fabricate open emitter NPN bipolar transistors for use in a bandgap reference circuit such as that taught in U.S. Pat. No. 4,287,439. To create a stable temperature independent CMOS bandgap voltage with amplifying means, such as an operational amplifier, two transistors of varying current density were used as emitter followers having resistors in their emitter circuits from which a differential voltage was obtained. An output voltage having a positive, negative or zero coefficient was thereby produced.
Several factors in the CMOS circuit, however, affected the initial tolerance variation and temperature variation of the bandgap voltage. The dominant initial tolerance error was caused by the offset voltage associated with the operational amplifier being multiplied by the ratio of two resistors in the emitter circuit of the transistor with lowest current density. Further disadvantages of the prior art are problems with P-resistor matching and a 2:1 variation in the P-resistivity over temperature. Previous CMOS bandgap circuits also required a startup circuit.